This invention relates to improvements in the suspension system for a vehicle, and is specifically related to controlling the disposition of the vehicle body relative to the vehicle wheels when the vehicle is subject to load distribution variation.
In recent times there has been a trend towards resilient sprung suspension systems incorporating variable damping and spring rates in an attempt to improve vehicle stability and reduce movement of the vehicle body relative to the surface being traversed. Some more advanced suspension systems, commonly referred to as active and semi-active suspensions, incorporate a number of electronic sensors which monitor information such as vertical wheel travel and body roll, as well as speed, acceleration, steering and braking commands. This and other data is processed by a computer which instructs hydraulic or pneumatic actuators to override the normal function of resilient springs in order to interpret, compensate and adjust the suspension's performance to suit speed, terrain and other factors in order to maintain a level ride and controlled distribution of weight onto all wheels. These suspension systems require an external intelligent back-up system, and call for substantial input of external energy, drawn from the vehicle engine, to operate the actuators that affect the adjustment to the suspension system.
A range of constructions of `active` and `semi-active` suspensions for vehicles have been proposed including systems operating on the basis of compression and/or displacement of fluids and such systems currently in use incorporate a pump to maintain the working fluid at the required pressure and effect the high speed distribution thereof, and sophisticated control mechanisms to regulate the operation of the suspension system in accordance with sensed road and/or vehicle operating conditions. These known systems incorporating pumps and electronic control systems, which are both required to operate substantially continuously while the vehicle is in operation, are comparatively expensive to construct and maintain, and require energy input, and therefore have limited acceptability in the vehicle industry.
There is disclosed in Australian Patent Application No. 65015/90, a vehicle having a load support body, and a pair of front ground engaging wheels and a pair of rear ground engaging wheels connected to the body to support same, and wherein each wheel is displaceable relative to the body in a generally vertical direction. Interconnected between each wheel and the body is a fluid ram including first and second fluid filled chambers that vary in volume in response to vertical movement between the respective wheels and the body.
The first chambers of the front and rear wheels on each side of the vehicle are in communication by respective individual first fluid circuits. Similarly the second chambers of the front wheels and of the rear wheels are in communication by respective individual second fluid circuits. This construction provides, when the vehicle is in use, substantially the same fluid pressure in the two chambers of any individual fluid circuit thereby inducing all wheels to maintain tractive ground engagement. In practice at least one and preferably each of said individual fluid circuits include at least one pressure accumulator, and preferably also a damping device operable to at least partially dissipate pressure shock in the fluid circuit or circuits.
The vehicle suspension above described differs greatly from all the known systems in that the wheel travel is not dependent upon progressive resilient suspension mechanisms which require variable reactions to the many ever changing conditions experienced by the vehicle. This allows free vertical travel of the individual wheels with respect to the vehicle body or chassis without having to first overcome the resistance of the conventional springing mechanisms normally incorporated between the wheels and the vehicle body. Thus, the wheels are individually unrestrained and free to move to follow the undulations of the surface being travelled without continually changing the vehicle weight distribution between the individual wheels. This reduction or elimination of changes in weight distribution significantly improves the traction between the wheels and the surface being traversed and the handling characteristics of the vehicle.
A further development of the above described suspension system is disclosed in Australian Patent Application No. 23664/92. In that suspension system a front wheel ram and the diagonally opposite rear wheel ram have the upper chamber of the front ram interconnected with the lower chamber of the rear ram and the lower chamber of the front ram interconnected to the upper chamber of the rear ram. Similarly the respective chambers of the other front ram and rear ram are likewise interconnected. There is thus provided two individual fluid circuits, each comprising a front ram and a diagonally opposite rear ram. Each of the conduits interconnecting the respective upper and lower chambers has a conventional pressure accumulator in communication therewith. The two circuits are interconnected to the load distribution unit arranged to maintain equi-pressure in the two circuits as is described in detail in the previously referred to Australian Patent Application No. 23664/92.
As most vehicles are non-symmetrically loaded for a large portion of the operating time thereof such that loads carried are located so the rear wheels carry more weight than the front wheels, or the load is closer to one side than the other, thus causing the vehicle body to tilt towards the heavier side or end.